As a steel tube used in a manufacturing plant of an ethylene, there has been known a so-called internal finned tube forming a plurality of (normally eight to twelve) fins in which a cross sectional shape (a cross sectional shape perpendicular to an axial direction of a tube) has a triangular round thread ridge shape and extending in the axial direction of the tube (a shape which is in parallel to the axial direction of the tube or a sharp which is inclined to the axial direction of the tube) in an inner surface thereof, for the purpose of increasing a heat transfer efficiency.
FIG. 1 is a cross sectional view schematically showing an example of the internal finned tube mentioned above. As shown in FIG. 1, an internal finned tube P is formed in its inner surface by crest portions (fins) M and trough portion R which are alternately provided in a circumferential direction of the tube. The internal finned tube P as mentioned above is normally manufactured in accordance with a centrifugal casting method and a hot extrusion tube manufacturing method which is typified by Ugine Sejournet process, by using a Fe base alloy having high Cr and high Ni as a raw material.
However, in the case of manufacturing the internal finned tube P by the hot extrusion tube manufacturing method, since the Fe base alloy having high Cr and high Ni serving as the raw material is inferior in a hot working performance, there is a characteristic that a shape of the crest portion M, particularly a shape of its top portion is hard to come to a predetermined shape. Accordingly, a countermeasure, for example, enlarging an extrusion ratio or the like is applied for making the shape of the crest portion M in the predetermined shape, however, in this case, there is a case that a micro crack-like defect K extending in an axial direction of a tube is generated in the trough portion R (particularly at the appropriately center of a trough bottom portion Rs), and a defect such as a fold flaw, a crack or the like is generated in the crest portion M.
Further, in the case of manufacturing the internal finned tube P in accordance with a cold rolling by a mandrel having a fin which is aligned with the shape of the fin formed in the inner surface by using a cylindrical raw tube, there is a case that a defect such as a crack, a sticking or the like in the cold rolling is generated in the crest portion M or the trough portion R.
If the generation of the defect as mentioned above is missed out on, it comes to a factor causing a severe accident during use of the tube P. Accordingly, it is necessary to take a measure for inspecting before shipping a product so as to fix up and remove the defect, and a non-destructive inspecting method having a high efficiency is desired.
Conventionally, there have been proposed an ultrasonic testing method, a fluorescent penetrant testing method, an eddy current testing method and the like as the non-destructive inspecting method detecting the defect generated in the inner surface of the internal finned tube.
As the ultrasonic testing method, for example, in Japanese Unexamined Patent Publication No. 10-274643, there has been proposed a method of making a shape echo from a side surface of the crest portion M forming an obstacle to identification of a defect echo from a defect K extremely small, as shown in FIG. 5. Specifically, there has been proposed a method of inputting an ultrasonic beam from an outer surface side of the tube at an angle of incidence θ (θ=90 to 70 degrees) in such a manner that the ultrasonic beam is approximately orthogonal to a diameter line L of the tube passing through the center of the trough bottom portion Rs, with respect to the center of the trough bottom portion Rs in the inner surface of the internal finned tube P.
The method described in the publication mentioned above generates no problem in the case that the inner surface shape of the internal finned tube P is approximately uniform in a circumferential direction of the tube, specifically in the case that a thickness t (see FIG. 1) of the trough bottom portion Rs is approximately even. However, if the inner surface shape is ununiform in the circumferential direction of the tube, that is, the thickness t of the trough bottom portion Rs is uneven, it becomes hard to identify the shape echo from the side face of the crest portion M and the defect echo from the defect K, and there is a disadvantage that it is not possible to detect the defect K at all, in the case that the unevenness of the thickness t is significant.
Further, in Japanese Unexamined Patent Publication No. 11-211704, there has been proposed a technique inputting an ultrasonic beam from a outer surface of a tube at an angle (90 to 70 degrees) in such a manner that the ultrasonic beam is approximately orthogonal to a diameter line of a tube passing through the center of the trough bottom portion with respect to the center of the trough bottom portion of the inner surface of the internal finned tube and detecting a defect by using an image processing. Specifically, there has been proposed a technique displaying by B scope after binarizing a testing signal detected by inputting the ultrasonic beam so as to separate into plural levels of signals, and detecting the defect generated in the trough bottom portion by image processing the B scope display image.
In accordance with the technique described in the publication mentioned above, it is possible to identify the defect echo and the shape echo on the image, in the case that a depth of the defect is large. However, in the case that the depth of the defect is small, it is hard to identify both the echoes and there is a risk of wrongly determining the defect. Further, since the imaging and the image processing are necessary, there is a disadvantage that it is hard to apply to a high speed inspection, and a cost of the processing apparatus becomes high.
Further, as the fluorescent penetrant testing method, for example, there has been proposed the following method, in Japanese Unexamined Patent Publication No. 2001-33393. In other words, first, a nozzle is inserted to an inner surface of an internal finned tube, a fluorescent penetrant fluid is sprayed from the nozzle, and the fluorescent penetrant fluid is applied to an inner surface of a whole length of the internal finned tube. Next, after a predetermined penetrating time of the fluorescent penetrant fluid has passed, the other nozzle is inserted to the inner surface of the internal finned tube, and water or a cleaning fluid is sprayed from this nozzle. Accordingly, the fluorescent penetrant fluid penetrating to the flaw is left in an inner portion of the flaw, and an extra fluorescent penetrant fluid attached to the inner surface is removed. Next, after drying the inner surface of the internal finned tube from which the extra fluorescent penetrant fluid is removed, an ultraviolet light including a visible light is radiated to the inner surface of the internal finned tube while inserting an inspection head. Further, the radiated inner surface of the tube is photographed by a camera head provided in the inspection head, and a determination is made whether the defect is present by an image reflected on a monitor.
Since the method described in the publication mentioned above requires the work taking a lot of trouble such as the applying work, the removing work and the drying work of the fluorescent penetrant fluid, there is a disadvantage that the method is not suitable for the inspection in which a high efficiency is demanded.
Further, since the eddy current testing method can efficiently detect the defect existing in the detected surface, the eddy current testing method is widely used as the non-destructive inspecting method of the inner surface of the tube. However, in the general eddy current testing method using a so-called internal inserted coil, since a distance between the internal inserted coil and the trough bottom portion of the internal finned tube becomes long, it is hard to detect a micro defect existing in the trough bottom portion.
Accordingly, for example, in Japanese Unexamined Patent Publication No. 58-166257, there has been proposed an eddy current testing method using a so-called external reference type coil, the method scanning an eddy current testing probe structured such that a detecting coil is embedded in a leading end portion and a reference coil is embedded at a position which is at a suitable length away from the detecting coil, along a spiral groove (corresponding to the trough portion of the internal finned tube).
However, in the eddy current testing method using the external reference type coil as described in the publication mentioned above, a noise tends to be generated due to a liftoff fluctuation of the detecting coil, ununiformity of the inner surface shape of the tube and the like, and there is a risk that the S/N ratio is lowered and the defect is missed out.
Further, in Japanese Unexamined Patent Publication No. 4-290950, there has been proposed an inner surface detecting head of an inner helically finned tube. The inner surface detecting head carries out eddy current testing by using a guide doubling as an eddy current testing sensor around which a coil for the eddy current testing is wound around a pair of fins respectively engaging with the trough portions of the tube and opposing to each other at the same time of being provided for a visual observation by imaging a tube inner circumferential surface reflected to a pyramidal mirror provided so as to be rotatable around a tube axis by a TV camera. Further, in the publication mentioned above, there is described that the defect existing in the trough bottom portion of the tube is detected by the eddy current testing sensor.
However, since a surface area of the coil is large in the coil as described in the publication mentioned above, that is, the coil wound around the spiral trough portion of the tube, the coil is not suitable for detecting the micro defect.